It is well-known that windows of vehicles can be prone—particularly when the weather is cold—to becoming foggy. The reason for foggy car windows has to do with air temperature, either inside or outside of the vehicle, and the air's moisture content. Specifically, water vapor in the air within a vehicle's cabin condenses on the interiors of the windows when the temperature of the air next to the windows drops below a specific temperature, called the dew point—the temperature below which water droplets begin to condense and dew forms. The dew point temperature increases as humidity in the air increases. In other words, when car windows are colder than the dew point temperature, which will rise due to warm, humid air inside the car, the air close to the windows cools to below the dew point temperature as well, causing the water vapor in the air to condense and stick to the windows, causing fog. Thus, on a cold day, any moisture in the air within the vehicle's cabin—from excessive body heat, passengers exhaling, damp clothes, snow on the floormats, etc.—can turn to condensation when the warm, moist air meets the air next to the colder surfaces of the windows that is below the dew point temperature.
Many current vehicles are equipped with sensors, often mounted on the windshield, for detecting the presence of moisture or fog on the interior of a window. Such vehicles typically use a relative humidity sensor or an infrared sensor to know when moisture has formed. In response to detecting the presence of moisture, a heating, ventilating, and air conditioning (HVAC) system equipped in the vehicle can be triggered and automatically activated to defog the window, i.e., remove the moisture from the window's interior.
Conventionally, one of these sensors will be mounted at a central location on the vehicle's windshield. However, a single sensor that is centrally located will be unable to detect condensation on the edges of the windshield. A possible remedy is to install multiple sensors along the windshield to detect condensation at any area, but conventional capacitive or infrared humidity sensors are typically expensive, requiring excessively high costs to implement.